The subject matter of the application deals with acquisition of High Performance Waveforms (“HPW”).
In order for a receiver to glean the information from a transmitted signal, the carrier frequency and phase of the signal must be determined and or synchronized. Typical transmitter and receiver pairs have matched expected transmit and receive frequencies. Thus signals sent by a transmitter possess a carrier frequency known or expected by associated receivers. The receiver pass band must be wide enough to accommodate not only the information-bearing signal, but also any fluctuation in the carrier, due perhaps to Doppler shift or drift in the transmitters frequency reference. Such transmitter errors and propagation characteristics can cause a frequency offset to be present.
Doppler shift is a characteristic of propagation that commonly produces a frequency offset as perceived by the receiver from the nominally transmitted frequency caused by relative motion of the transmitter and receiver. Ignoring higher order effects, the value of the frequency offset )f, is given by Vf0/c, where V is the relative velocity, f0 the nominal frequency and c the speed of light.
This requirement in the receiver pass band results in additional noise energy passing over and above the amount theoretically required by the bandwidth of the information. Thus the frequency offset can result in a deterioration of the signal to noise ratio (SNR) to a degree that the signal is not easily distinguishable from non-signal noise by the receiver. The frequency offset additionally can reduce the capability of the receiver to retrieve the information contained in the signal.
Complicated receivers that employ a carrier frequency tracking loop are able to keep a narrow passband filter centered about the carrier, thereby substantially reducing the detected noise energy and improving the received SNR. However, large frequency offsets can prevent the tracking loop from even obtaining the signal.
Another consequence of an offset that occurs from time to time, especially for high frequency offsets between transmitter and receiver, is a phenomenon known as a “click”, which leads to a high estimation error of the carrier phase and frequency. Through sampling of the signal, complex samples travel a complete circle around the origin of the imaginary plane. However, if the influence of noise is significant enough, the path of the samples might not include the origin, this phenomena resulting in the accumulation of the phase missing 2B, or a click.
The subject matter in this application addresses the detection of large frequency offset signals with a preamble of frequency modulated Maximum Length Sequences (MLS's) and a predetermined period of pure carrier along with probable frequency offsets to widen the acquisition bandwidth. The subject matter described herein also addresses the detection and correction of clicks.
Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art and to provide a novel method of wide band acquisition of a high performance waveform, in an environment with poor signal to noise ratio. The waveform having a preamble with a plurality of frequency modulated Maximum length sequences and a period of pure carrier. In one embodiment, an inventive method provides an application specific integrated circuit (ASIC) for receiving the waveform, filter coefficients for use with the MLSs portion, and filter coefficients for the pure carrier portion of the signal. This method may also include for the MLS portion of the signal, detecting the signal through partial correlation, extraction of waveform information, and estimation of symbol timing. The method may further include for the pure carrier portion of the signal, estimating the phase and frequency and providing such estimates to the ASIC.
It is another object of the present invention to provide a novel system and method for detecting a communication signal in an environment with poor signal to noise ratio, the signal including a preamble with a plurality of frequency modulated MLS and a predetermined period of pure carrier. In another embodiment, the method involves receiving a candidate signal, sampling the candidate signal a plurality of times for each symbol, and calculating a weighted frequency. From the weighted frequencies a 1st and 2nd attribute one calculated and the attributes are compared to determine whether a communication signal has been detected.
It is yet another object of the present invention to provide a novel system and method for detecting a narrow band communication signal in an environment with poor signal to noise ratio, the signal including a preamble with a plurality of frequency modulated MLS and a predetermined period of pure carrier. A further embodiment of the present invention may be a system and/or method of applying the candidate signal to a first mixer tuned to the expected carrier frequency to obtain a first intermediate signal, and supplying the first intermediate signal to a central processing branch, comprising x number of upper parallel processing branches and y number of lower parallel processing branches. The processing branches, sample a plurality of times each symbol in the intermediate signal, calculate a weighted frequency, calculate a 1st and 2nd attribute and compare the attributes to determine whether a communication signal has been detected.
It is still another object of the present invention to provide a novel system for detecting a communication signal with a large frequency offset in an environment with a poor signal to noise ratio, the signal with a preamble including a plurality of MLS sequences and a pure carrier signal. A still further inventive system includes a first mixer and first filter, a central processing branch, comprising x number of positive processing branches and y number of negative processing branches for processing an intermediate signal. Each processing branch may include a limiter for removing a DC offset, a correlator and a logic circuit. The system may also include an adjustable first filter and an adjustable first mixer one or both of which may be adjusted with output of logic circuits.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.